The present invention pertains to the art of acoustics, particularly ultrasonic inspection of workpieces. The present invention finds particular application in the inspection of cylindrical, tubular goods to detect flaws, measure wall thickness, and monitor other physical properties. It is to be appreciated, however, that the invention finds further application in the inspection of other workpieces of various sizes and shapes for the above and other purposes.
Heretofore, various inspection systems have been used to inspect pipes and other tubular goods. These prior art inspection systems have included X-radiation inspection systems in which variations in the amount of X-radiation passing through the pipe indicated areas of wall thickness, flaws, and the like. The prior art also included magnetic flux leakage inspection systems in which variations in a magnetic flux leakage field indicated flaws or other non-uniformities in metal tubing. The X-radiation and magnetic flux leakage type tubing inspection systems tended to provide a relatively coarse examination of the tubular product. Once a flaw or thin spot was detected, it was commonly re-examined with a hand held ultrasonic inspection apparatus. Commonly, the hand held ultrasonic apparatus was calibrated by physically measuring the wall thickness in a convenient area with a mechanical gauge, such as a micrometer or the like. The ultrasonic apparatus was then operated to transmit a longitudinal wave through the measured wall thickness. An ultrasonic wave travel time in which the wave traversed the measured thickness is directly proportional to the thickness. The conversion of the travel time to thickness was adjusted or calibrated such that the ultrasonically measured thickness matched the mechanically gauged thickness.
One of the problems encountered with ultrasonics in measuring dimensions and other physical properties is that the velocity of the ultrasonic wave varies significantly with subtle differences in the physical structure of the workpiece. The acoustic wave velocities vary with such physical properties as elastic moduli and density which, in turn, may be a function of metallurgical composition and manufacturing process variations, and the like. For example, the acoustic velocity of ultrasonic waves in steel tubing will vary with such factors as the temperature to which the steel was heated at the mill, the rate at which the steel was cooled, the steel composition, impurities and variations in the steel components, and the like. It has been found that the acoustic velocity of ultrasonic waves in steel tubing meeting common specifications varies as much as 400% from one mill to another. Further, the acoustic velocity varies with the batch of steel from which the tubing was manufactured, and may even vary within a single workpiece. Accordingly, in order to automate and insure accurate measurements, frequent recalibration of the ultrasonic instrument is necessary to compensate for acoustic velocity variations in the product under examination.
The present invention contemplates a new and improved acoustic inspection system which overcomes the above referenced problems and others.